A polyurethane resin is produced usually by injecting into a mold and reacting a polyol and a polyisocyanate in the presence of a catalyst and, if necessary, various additives such as a chain extender. The polyurethane resin is widely used as a flexible foam for e.g. seat cushions for automobiles, mattresses, furnitures, etc., a semirigid foam for e.g. instrument panels for automobiles, headrests, armrests, etc, or a rigid foam to be used for e.g. electrical refrigerators, building materials, automobile interior materials, etc. Further, as compared with rubber shoe soles or ethylene/vinyl acetate copolymer (EVA) shoe soles, the polyurethane resin is excellent in abrasion resistance and has a merit such that feet are less likely to get stiff during walking, and its production process involves a less load as compared with shoe soles made of other materials, and therefore, it is widely used for shoe soles.
In recent years, in the production of a polyurethane resin for shoe soles, an excellent curing rate to improve the productivity and excellent moldability to improve the yield have been strongly demanded with a view to saving energy or reducing costs. Further, in order to let the material liquid reach every corner efficiently at the time of injecting the material liquid into a mold, the polyurethane resin is required to have an excellent flowability, and in order to secure the lidding time of the mold as long as possible, it is required to suppress the initial reactivity. However, even if the initial reactivity is suppressed, it is required to facilitate the curing more than ever in order to improve the productivity.
The reaction for forming a polyurethane resin comprises mainly a urethane group-forming reaction (gelling reaction) by a reaction of a polyol with an isocyanate and/or an isocyanate prepolymer, and a urea group-forming reaction (blowing reaction) by a reaction of an isocyanate and/or an isocyanate prepolymer with water. The reaction for forming a polyurethane resin for shoe soles includes, in addition to the above two types of reaction, a crosslinking reaction by a reaction of a crosslinking agent with the urethane group or a reaction of a crosslinking agent with the urea group. The catalyst gives substantial influences not only on these reaction rates but also on the curing rate, flowability, moldability, dimensional stability and physical properties of the polyurethane resin, etc.
As the catalyst for production of a polyurethane resin for shoe soles, a catalyst to accelerate particularly the reaction (gelling reaction) of a polyisocyanate with a polyol and/or the reaction (blowing reaction) of a polyisocyanate with water, has been preferred, and it is widely known that a tertiary amine catalyst becomes an excellent catalyst for production of a polyurethane resin (e.g. Patent Document 1). However, in a case where such a tertiary amine compound is utilized in the production of a polyurethane resin for shoe soles, it has been difficult to shorten the curing time while delaying the time to initiate the reaction, and a solution to this problem has been desired.
For example, a catalyst having a part of the tertiary amine protected by formic acid, or a catalyst for production of a polyurethane composed of a specific saturated dicarboxylic acid and the tertiary amine has been proposed (e.g. Patent Document 2). However, if the initiation time is delayed, the curing time will also be delayed, and the above problem has not thereby been solved.
Further, many cases have been disclosed wherein a quaternary ammonium salt type catalyst is utilized as a catalyst for production of an urethane shoe sole resin, for example, Patent Document 3 discloses use of an N,N′-bis(hydroxyalkyl) quaternary ammonium salt of triethylenediamine, and Patent Document 4 discloses use of triethylenediamine, a quaternary hydroxyalkyl base of imidazole, and their salts. However, if it is attempted to secure the delayed action in curing by using these quaternary ammonium salt-type catalysts for the production of polyurethane resins, there have been problems such that the curing property of the obtainable polyurethane resin tends to be poor, and the final curing tends to decrease or be deteriorated, and therefore a solution of such problems has been desired.
Further, a metal salt, a particularly an alkali metal carboxylate, is well known as a catalyst to accelerate particularly a polyisocyanurating reaction (trimerization of a polyisocyanate). However, when it is used for formation of a polyurethane resin for shoe soles, it has been difficult to control the reaction rate, and the strength of the obtainable polyurethane resin has been inadequate, whereby it has been difficult to obtain a shoe sole product having good moldability.
An organic tin-type catalyst, e.g. dibutyltin dilaurate, is frequently used in a method of employing a polyether polyol developed for the purpose of improving the hydrolyzability of an urethane shoe sole resin, for the reason that the curing property is thereby improved. However, due to a toxicity problem of organic tin, a demand for a substitute catalyst has been increasing.
Further, it is also known to use 1,8-diazabicyclo[5.4.0]undecene-7 as a highly temperature sensitive catalyst for forming a polyurethane resin, but like the alkali metal carboxylate, it has been difficult to control the reaction rate and the strength of the obtainable polyurethane resin is inadequate, whereby it has been difficult to obtain a shoe sole product having good moldability. Further, 1,8-diazabicyclo[5.4.0]undecene-7 is susceptible to hydrolysis, and it is necessary to suppress hydrolysis by using a blocking agent such as phenol, but use of phenol is restricted because of its high toxicity, and a voice to demand a substitute catalyst has been strong.
Further, a catalyst for production of a polyurethane which comprises triethylenediamine and at least one selected from the group consisting of 1,8-diazabicyclo[5.4.0]undecene-7,1,5-diazabicyclo[4.3.0]nonene-5 and their salts, has been proposed (e.g. Patent Document 5). According to Patent Document 5, with a catalyst comprising triethylenediamine and 1,8-diazabicyclo[5.4.0]undecene-7, it is possible to overcome a drawback that the transparency of the obtained shoe sole deteriorates as the time passes, but the above-mentioned problems have not yet thereby been solved.
Patent Document 1: JP-A-62-233102
Patent Document 2: JP-A-2000-95831
Patent Document 3: JP-A-61-207420
Patent Document 4: U.S. Pat. No. 3,010,963
Patent Document 5: JP-A-2005-206800